In general, batteries are devices that convert chemical energy into electrical energy, by means of an electrochemical oxidation-reduction reaction. Batteries are used in a wide variety of applications, particularly as a power source for devices that cannot practicably be powered by centralized power generation sources.
Batteries can be generally described as comprising three components: an anode that contains a material that is oxidized during discharge of the battery; a cathode that contains a material that is reduced during discharge of the battery; and an electrolyte that provides for transfer of ions between the cathode and anode. Batteries can be more specifically characterized by the specific materials that make up each of these three components. Selection of these components can yield batteries having specific voltage and discharge characteristics that can be optimized for particular applications.
Batteries can also be generally categorized as being “primary,” where the electrochemical reaction is not reversed, so that the battery becomes unusable once discharged; and “secondary,” where the electrochemical reaction is, at least in part, reversible so that the battery can be “recharged” and used more than once. Secondary batteries are increasingly used in many applications, because of their convenience (particularly in applications where replacing batteries can be difficult), reduced cost (by reducing the need for replacement), and environmental benefits (by reducing the waste from battery disposal).
Among the most common secondary battery systems are lead-acid, nickel-cadmium, nickel-zinc, nickel-iron, silver oxide, nickel metal hydride, rechargeable zinc-manganese dioxide, zinc-bromide, metal-air, and lithium batteries. Systems containing lithium and sodium afford many potential benefits, because these metals are light in weight, while possessing high standard potentials. In fact, contemporary portable electronic appliances rely almost exclusively on rechargeable lithium (“Li”)-ion batteries as the source of power, because of their high specific energy, high cell voltages, and long shelf-life.
Li-ion batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (or electroactive) materials. Among such batteries are those having metallic lithium anodes and metal chalcogenide (oxide) cathodes, typically referred to as “lithium metal” batteries. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous aprotic organic solvents. Other electrolytes are solid electrolytes that contain an ionic conductive medium (typically a lithium containing salt dissolved in organic solvents) in combination with a polymer that itself may be ionically conductive but electrically insulating.
A variety of materials are used commercially as the cathode active materials in Li-ion batteries. Of these, olivine-type LiFePO4 is commonly accepted as one of the most promising cathode material for large-scale Li-ion batteries because of its low-cost, non-toxicity, and extremely high stability. However, this cathode material has very poor conductivity, and also has a low theoretical specific energy of ˜530 Wh/Kg due to a low operating voltage of ˜3.4 V. Compared to LiFePO4, LiCoPO4 with higher operating voltage (4.7 V) is predicted to have ˜1.5 times the specific energy of LiFePO4. Moreover, LiCoPO4 is also expected to have higher electrical and lithium ion conductivity than LiFePO4 potentially enabling the achievement of an even higher volumetric energy density by using thicker electrodes, less carbon coating, or larger particle size. In addition, olivine-type cathode materials are generally safer than more commonly used LiTMO2-type cathodes because of their greater thermal stability resulting from the strong covalent bonding between the oxygen and P5+ to form (PO4)3− tetrahedral polyanion. However, LiCoPO4 materials generally have low electronic conductivity and relatively poor Li+ diffusivity, which causes low charge/discharge capacity and large degradation of capacity. Several approaches have been used to improve performance of olivine-type cathodes, such as coating with electrically conductive materials (typically carbon), reducing particle size and doping. However, due to differences in their Li-electrochemical behavior, the approaches used to improve conductivity cannot be universally applied to all materials. To date, cathode materials comprising combinations of Fe and Co as dopants in lithium materials have shown relatively poor battery properties relative to both LiFePO4 and the theoretical performance. (FIG. 1)
Ideally, a Li-ion cathode material would exhibit a high voltage potential and capacity, the ability to be recharged over multiple cycles with high efficiency, and would be economically practical to produce. However, many of the cathode materials known in the art lack one or more of these characteristics. As new and existing battery applications demand continuous improvements in battery capabilities, there is an unmet need for battery materials that can provide desirable battery properties.